In 3769ed6c66 we added handling for SSH version 1.99 which unsed a SSH
version of 0 to indicate weird cases where no version could be
determined.
This patch is a fixup for that patch. Instead of using a magic version
of 0 we now use an `&optional` version value. If no SSH version can be
extracted the version will be unset; additionally a `conn_weird` event
will be raised.
Closes#1590.
